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WWII Scrambler Phone

The Secraphone, or Scrambler Phone, was a secure telephone system issued by the General Post Office (GPO) in the UK around 1937. It consists of telephone set No. 394 or No. 396, and a rather large external box made by TMC, that contains the actual voice scrambler. The 394/396 set acts as the voice terminal and looks like a normal telephone. It often has no dial, features two special switches marked SECRET and ENGAGE FOR SECRET 1 and can be identified by a green handset.
During World War II, most of the telephones sets were manufactured by TMC and Plessey. The set supplied with Secraphone has a black bakelite body and is fitted with a green bakelite handset, as shown in the image on the right. Whenever green handsets were in short supply during the war, standard black ones were painted green.

The voice terminal does not have a dial, as the call was normally initiated elsewhere. Instead of the dial, a blanking panel (No. 3) it usually fitted, often with a circular paper note at the rim that says: SPEECH ON TELEPHONES IS NOT SECRET.
Secraphone voice terminal

At the center of the blanking panel is a circular area that can be used for holding the extension number and for a short instruction on how to initiate a call. Above the blanking panel, just below the handset, are two buttons that are used to switch between SECURE and CLEAR modes. They are commonly labelled SECRET and ENGAGE FOR SECRET as shown above, or words to that effect.
Telephone No. 394 was manufactured by British Ericsson, Plessey, Pye-TMC, and perhaps others. It was introduced by the General Post Office (GPO) in 1937. Quite a few of these telephones with green handsets are now on public display at the Churchill War Rooms 2 in London (UK) [17].

The WWII image on the right shows British Prime Minister Winston Churchill behind his desk at the Cabinet War Rooms, just 10 feet below street level under the New Public Offices 3 at Whitehall. Next to him is Royal Navy Captain Richard Pim operating the Secraphone terminal on his desk.
Churchill in the Cabinet War Rooms during WWII, with a staff member operating his Secraphone. Copyright IWM [13].

In 1938, after a survey by the Office of Works, this building was thought to be suitable for use as a temporary office in the event of war. It was hastily converted into a temporary command center and became operational in August 1939, just before the outbreak of WWII. Although the 'bunker' was reinforced several times during the war, it hardly offered any protection against bombs.
The actual voice scrambler was a large valve-based device that was know as the Privacy Set. The initial one, Privacy Set No. 6 4 , was not very secure as it was just a simple-frequency inverter. It was later replaced by Privacy Set No. 7 which featured frequency and time domain scrambling. In 1955, these sets were succeeded by the much small and fully transistorised Privacy Set No. 8. 5
  1. Other descriptions for the buttons are also known. See below for a full list.
  2. The Churchill War Rooms consist of the Churchill Museum and the Cabinet War Rooms.
    They are now part of the Imperial War Museum (IWM) and are open to the public [17].
  3. This building now houses the Treasury.
  4. Privacy Set No. 6 was also known as Frequency Changer No. 6 and Privacy Unit No. 6.
  5. Privacy Set No. 8 was also known as Secrecy Unit No. 8.

Secraphone voice terminal Front view Two Secraphone voice terminals Telephone No. 396 used as Secraphone voice terminal Close-up of the 'dial' and the push buttons Interior bottom view Drawer closed Secraphone terminal with open drawer

Telephone No. 394 and 396
The original Secraphone consisted of two parts: a rather heavy rectangular box that contains the actual valve-based voice scrambler, or privacy set, and a converted standard telephone set that was used as the voice terminal. The latter was based on the 300-series GPO phones of the era.
The telephone set was developed by the GPO in cooperation with Ericsson Telephones Ltd (UK), and the first model to be introduced in 1937 was the 332. It was followed by the 330 and the 312, each with a single button, the 'blind' 332 CB, and multi-button versions like the 394 and 396.

The 394 and 396 sets could be fitted with 2 or 3 buttons that could be configured for a variety of tasks like recall, lamp on/off, bell on/off, party line, ring extension, etc. For use in combination with the Privacy Set, only the outer two buttons were needed and the center hole was covered.
Close-up of the push button on the phone

The rightmost button is the only one that has a latching feature. When it is pressed, the phone is connected directly to the line. Pressing the leftmost button, releases the rightmost one. In this state, microphone and speaker are routed via the voice scrambler. Some phones were modified, so that the buttons were also released when the handset was placed in the cradle (on-hook).
When the 300-series telephone was developed, the existing handset No. 164 [16] from the older 200-series was used again. The telephone body and the handset were both made of bakelite and were available in black (most common), lacquer red, jade green and ivory. For use with a Privacy Set, a black body was used with a green handset.

During WWII the increased demand for scrambler phones sometimes caused shortages of green bakelite handsets. As an alternative, standard black handsets were then painted green, or were issued with a green mouthpieces and earcaps.
Green painted handset

Likewise, when the 394 telephone sets were in short supply, the 396 was used as an alternative. Although there are small differences between these two models, this was not a problem for the intended use. The set shown in the image above is a 396 with a painted handset, whilst the one at the top is a 394 with a green bakelite handset. Yet, they were both found in the same location.
Although a dial could be fitted to the 394 and 396 phones, they were usually issued without one, as it was not needed by the Secraphone system. In its place, a circular blanking panel was mounted, often with a warning to the user:


This was done to make the user aware of the fact that standard telephone lines were unprotected and that their conversations, especially during a war, were a target of enemy eavesdroppers. The Secraphone provided only limited protection.
Close-up of the 'dial' and the push buttons

The circular area at the centre of the blanking panel was commonly used for holding a card with the extension number, sometimes with a short instruction on how to initiate a call. The card was covered by a transparent plastic or celluloid disc and was held in place by a circular spring clip.

At the front of the phone, towards the bottom is a drawer that can be pulled towards the front. Inside the drawer is a hinged plastic holder that can be used to hold further instructions or telephone numbers. If the drawer is not present, this space is covered by a blanking panel.

 Telephone No. 394 Datasheet
 Telephone No. 396 Datasheet
 Handset No. 164 Datasheet
Secraphone voice terminal Close-up of the 'dial' and the push buttons Close-up of the push button on the phone Green painted handset Telephone No. 396 used as Secraphone voice terminal Close-up of the buttons and the 'dial' of the 396 Two Secraphone voice terminals Drawer open

Over the years, many Secraphone installations were issued, especially during World War II. As the required parts were often in short supply during the war, alternatives were sometimes modified and issued to users. For this reason, a wide variety of telephones and handsets can be found.

Two Secraphone voice terminals

The 394 telephone was used, as it had provisions for up to three buttons, but whenever it was in short supply, the 396 was used as an alternative. Both phones could easily be converted for use with a Privacy Set. Although a two-button arrangement is the most common one, a three-button variant is known to have existed. In that case, the third button (at the centre) was used for recall.

The handset was made of jade green bakelite, but the mouthpieces or ear caps were sometimes black, either because they had been replaced at some point, or simply because they were not available at the time the system was supplied to the user. In the same vein, black handsets with a green mouthpiece and ear cap have been found as well. If green bakelite handsets were in short supply, standard black ones were painted or sprayed green, like the rightmost one above. The colour of the paint used for this, does not match the colour of the green bakelite handsets.

Jade green GPO handset No. 164

The handsets usually had a green or brown braided cord, whichever was available. Note that the handsets already existed before the 300-series phones were introduced, as they were also used on the earlier 200 phones. As a result, the date marking on the handset can not be used to determine the age of the phone. Always use the date code on the chassis as a reference. One of the phones in our collection is from 1938, whilst the green bakelite handset was made in 1935.

There are also different variants of the metal shield with the text labels that is mounted just below the SECURE and CLEAR buttons. At least four version have been spotted over the years, which are shown below. In all cases, the leftmost button is used to go secure (secret, scramble), whilst the rightmost button is used for clear speech (normal, hold scrambler, engage for secret).

We believe the above label to be the eldest as it does not appear in the 1952 list of labels that was used at the GPO. Furthermore it is present on the phones in our collection that were made in 1938 and 1940 respectively. Other labels that are known to have been issued over the years are:

Depending on the configuration of the telephone set, the user requirements and the presence or absence of a third button at the centre, other arrangements and text labels may have existed. The labels could be engraved or screen printed. If it was screen printed, a condensed variant of the Gill-Sans typeface was commonly used. For a complete overview of the 27 different text labels No. 252 & 253 that were available between 1952 and 1967, please refer to list N620 [15].

 Overview of text labels

Although it was technically possible to fit a dial to a 394/396 telephone, the standard issue was without one and the large circular hole was covered with Blanking Panel No. 3. One some phones, a large circular label with a warning was glued to the outer rim of this blanking panel, but this was not always the case. Likewise, the circular label at the center was sometimes omitted as well.
Real or fake?
Genuine GPO No. 394 telephone sets that have been used as part of a Secraphone secure speech setup, are extremely rare and are very difficult to find. Many collectors have created a mockup by taking an existing multi-button 300, swapping the text shield and adding a green handset to it. If you are a collector and are searching the internet for genuine 394 scrambler phones, there are a few pitfalls to watch out for. Here are some examples of phones that have turned up in the past.
In June 2013, the telephone set shown in the image on the right turned up for sale at Giddings Auctioneers in Leicester (UK). According to the description it is a GPO Telephone No. 394, which had been used as part of a scrambler telephone system, in combination with Security Unit No. 8.

Although this set looks complete and in good condition, we have some reservations about its authenticity. There are strong indications that the set has been put together from spare parts and/or from parts from other telephone sets. First of all, the unit has a dial and, although it is perfectly possible, this was very uncommon for terminals that were used with the Secraphone.

Secondly, and more seriously, the two buttons, marked NORMAL and SECRET, are labelled the wrong way around. Although various designs of the text label are known, the SECURE button is always on the left and the CLEAR button on the right. This phone therefore seems to be a fake.

Telephone No. 394. Copyright Gildings Auctioneers [10].
Another example of a non-authentic scrambler phone is shown in the image on the right. The item appeared on eBay in February 2016, but in this case, the seller made it very clear in his description, that it was a reproduction [14].

According to the description, it consists of a Telephone set No. 328L of 1962 vintage, with a green reproduction handset that was added at a later date. The scramble/normal plate is genuine and was given to the seller in 1981 when he was an apprentice at the GPO/BT, by a former fitter who used to work in the Houses of Parliament.
Image of a reproduction Secraphone terminal as sold on eBay in February/March 2016 [14]

On both the above examples, the dials are really out-of-place, but might be necessary if you wanted to actually use them on a local PABX, like in the second case. If the dials were replaced by a genuine blanking panel No. 3, they would look excellent in any WWII Secraphone display.
Privacy sets
The initial voice scrambler was a valve-base frequency inverter, known as Privacy Set No. 6 1 followed by the improved Privacy Set No. 7 which was a frequency and time domain scrambler. Both sets remained in service throughout the war. Privacy Set No. 6 was used until the late 1940s and Privacy Set No. 7 even lasted until 1955 after which they were gradually replaced by the fully transistorised Privacy Set No. 8 3 often in combination with Telephone No. 710 or 740 [2].

  1. Privacy Set No. 6 was also known as Frequency Changer No. 6 and Privacy Unit No. 6.
  2. Privacy Set No. 8 was also known as Secrecy Unit No. 8.

Privacy Set No. 6
The Secraphone was initially based on the so-called Privacy Set No. 6, also known as Frequency Changer No. 6, a fairly simple voice scrambler based on the principle of frequency inversion. It contained a valve-based balanced ring modulator with a reference frequency of 2500 Hz and was used in combination with a black GPO telephone set No. 394 with green handset. All the party at the other end of the line needed, was an identical Privacy Set with a similar No. 394 telephone.

This system was in use between 1937 and 1949 (shown in the above timeline in green) and was rather unsafe as it was based on a simple principle and had no key settings. By mirroring the frequency around 2500 Hz once again, an eavesdropper could recover the original voice data.
Privacy Set No. 7
In 1940, at the outbreak of WWII, an improved version of the scrambler with better security was introduced. In addition to frequency inversion it also featured time domain scrambling (also known as F/T scrambling). The same GPO Model 394 phone was used as the voice terminal.
The set became known as Privacy Set No. 7 and remained in use until 1955 (shown in the above timeline in red), after which time it was gradually replaced by the transistorised Privacy Set No. 8.

Nevertheless, some Privacy Sets No. 7 and even some No. 6 units remained in use until at least 1962 [4][5]. It is believed that the device shown in the image on the right is a Privacy Set No. 7 and that the key settings are on the right [11]. The dial on the 394 telephone set seems to be misplaced, so this might be a post-war replica. The actual system seems to be authentic though.
Secraphone consisting of Privacy Set No. 6 and Telephone No. 394. [11]

It is sometimes suggested that for the improved Privacy Set No, 7, black telephone sets with red handsets were used, but this is by no means certain. It is far more likely that the British kept using black phones with green handsets for this. Red handsets probably had a different meaning. The image below shows an advert for the Secraphone and probably dates back to the 1950s [6].

Advert for the Secraphone by Telephone Manufacturing Co Ltd. (TMC). Via Sam Hallas [6].

Wireless Set No. 10
The improved Privacy Set No. 7 was sometimes used over multiplexed phone lines or even via radio channels. In the latter case, the so-called Wireless Set No. 10 or WS10 was used.

The WS10 was introduced during WWII in July 1944, shortly after D-Day, and allowed up to 8 individual voice channels to be relayed using Time Division Multipexing (TDM). In such cases the other channels were commonly filled with other voice traffic or simply with noise.

The image on the right shows how the WS10 was used in the period following the Allied Invasion of D-Day [12]. Carrying important voice calls through France, Belgium and The Netherlands, Montgommery referred to it as his 'No. 10 thing'.
Complete WS10 installation mounted on a trailer. Photograph via Louis Meulstee [12].

Privacy Set No. 8
In 1955, shortly after the invention of the transistor, the General Post Office (GPO) introduced a much smaller and more modern - fully transistorised - version of the Secraphone, which became known as Privacy Set No. 8.

The set was built by different manufacturers, including PYE/TMC and AGI [7], and worked in a similar fashion to the earlier valve-based units. It was generally used with a modified Telephone No. 710 or 740, but could also work with the older Model 394. The image on the right shows a Privacy Set No. 8 as it was auctioned in the UK by Gildings in 2013 [10].
Privacy Set No. 8. Copyright Gildings Auctioneers [10].

Telephone No. 710 and 740
Privacy Set No. 8 was generally supplied with a modified Telephone No. 710 or 740 as a suitable voice terminal. The image on the right shows a GPO No. 710 phone which was based on the current design of the mid-1950s.

Besides the Telephone 710, the Privacy Set No. 8 could also be used with the earlier Telephone No. 394, which was often the case when the older No. 6 and No. 7 sets were replaced by the modern No. 8.
Telephone No. 710. Copyright Gildings Auctioneers [10].

Getting access to the interior of a 394 or 396 telephone is quite straightforward. Loosen the four bolts at the corners of the bottom panel (not the rubber feet) and take it away. The bottom panel also holds the small drawer. Note that a circuit diagram should be present under the drawer.
Inside the set is a metal chassis to which all internal parts are mounted. Prominently visible towards the front is the optional bell which takes up most of the space. Apart from the bell, the bottom section also houses the transformer, a large capacitor and a 13-point contact block for connection of the handset and the outside line.

The chassis can be removed from the body of the telephone by loosening three bolts: two at the sides and one towards the rear (behind the capacitor). As the Secraphone has no dial, there are no wires that have to be disconnected.
Interior bottom view

The top side of the chassis holds the contact strip for the dial (not used here) and a large block with an array of switches. These are the switches that are operated by the two or three buttons that are located on top of the telephone set. In GPO terminology this is the so-called KEY UNIT.
As multiple contacts are needed for switching microphone and speaker, Key No. 303/A is used. It consists of three individual switches with 4, 1 and 4 sets of make-before-break contacts each. This arrangment is also known as 4K-1K-4K 1 , and the complete set is often referred to as 9K.

Towards the front of the key assembly are three plungers that are operated by the buttons on top of the phone. The behaviour of the plungers is controlled by a spring-loaded latch bracket that is fitted at the front. In the current Secraphone setup, only the rightmost switch is latched.
Close-up of the frame

Pressing any of the other buttons just releases the rightmost one. This is done to avoid operator mistakes resulting in accidental transmission of clear speech. As the rightmost button is used for switching to CLEAR speech, this means that when operating the phone, the user deliberately has to select CLEAR speech whilst the default setting of the phone is SECURE (all buttons released).
At the rear side of the No. 303A switch pack is a 28-point contact block with screw terminals, to which the actual contacts of the 9K (4K-1K-4K) switches are wired. This allows the switch pack to be used for virtually any configuration or application, simply by wiring it up as required.

The line cord and the braided handset cord, are usually fixated to one of the mounting posts of the terminal block (T), by means of tie knot. Note that the bell was optional on the 394 and 396 phones. It was only fitted when required by the customer. It is present in the phone shown here.
tie-knotting of the handset cable

At present, the correct wiring of the switch pack for use in combination with a Privacy Set is unknown, as the phones in our collection were rewired during the post-war years for back-to-back operation. Nevertheless, all parts are present and wiring it correctly should not be difficult. Note that the line cord shown above is not the correct one as it has only 3 wires. In the original setup, a multi-wire cable was used between the telephone set and the Privacy Unit.
  1. In Ericsson terminology, a single make-before-break set of contacts is known as a 1K springset. Likewise, a double set is known as 2K and a set of four make-before-break contacts is called 4K. More...

Interior bottom view Interior front/top view Interior rear/top view Interior bottom/rear view Interior bottom/front view Interior with handset tie-knotting of the handset cable Connection block (T) seen from the bottom of the phone
Close-up of the frame Hook switch engaged (on-hook) Top view of the 9K switch pack (303/A unit) Metal plate for 'programming' the behaviour of the three buttons Terminals for connection of the dial (not used here) 303/A key block (9K switch pack) Pressing the rightmost button Pressing the leftmost button
Bottom panel with drawer and circuit diagram Telephone No. 394LB circuit diagram inside the device Close-up of the circuit diagram No. 396 bottom panel with circuit diagram Close-up of the circuit diagram of the 396 Close-up of the bell Close-up of the transformer Capacitor mounting bracket. Note the date stamp on the capacitor (1940).

The Secraphone terminals shown on this page are authentic and have been used during WWII with a Privacy Set. According to the date stamps on the chassis, one unit was made in 1938, whilst the other one is of 1940 vintage. The green handset was made in 1935. The painted one in 1940.
The problem with these two sets however, was that a previous owner had converted them into house telephones, or intercoms, and used them this way for several years. Obviously he wanted to avoid the use of a small exchange, or PABX, and had converted them for low power use.

The original bell had been removed and its space was used to accomodate two 4.5V batteries: one for the speech loop and one for a small buzzer that was mounted to the chassis. Luckily, the previous owner had applied his modifications in such a way that they could easily be undone.
Lamp fitting mounted to the middle button

A bakelite button had been added to the center of the circular panel that covers the hole of the dial, but this too was easily removed. The bad news was that the batteries were left inside the phones when they were taken out of service. Over the years, the leakage from the batteries had caused considerable damage to the bottom panel and to the small drawer at the phone's front.
Restoration of the phones was started by first taking them fully apart and cleaning the indiviual parts. The modifications were removed and undone and the bakelite body of the unit was washed and treated with bakelite conditioner. 1

The missing parts were then re-mounted to the chassis and the original wiring was restored according to the circuit diagram at the bottom. The braided cord of the green bakelite handset was replaced with a high-quality reproduction 2 and a new braided line cord was added to allow the phone to be connected to a standard line.
Rust caused by leaking batteries

Both phones are now fully restored as close as possible to their original state. The only thing missing right now are two original Privacy Sets to connect them to, so that we can finally demonstrate how Churchill held private phone conversations during the war.
Before restoration Lamp fitting mounted to the middle button Bakelite doorbell knob mounted at the center Telephone No. 394 before its restoration Bakelite body after restoration Inside the bakelite body Rust caused by leaking batteries

  1. High-grade bakelite conditioner and other products for restoring bakelite parts are available from Ron van Zaltbommel in The Netherlands.
  2. High-quality reproduction cables for old GPO phones, that closely match the original colours and manufacturing properties, are available from Chris Elliot in the UK.

An excellent documentary about Churchill's Cabinet War Rooms is available on YouTube via the link below. It features a combination of archive film material from the Imperial War Museum's vast collection, and atmospheric dramatisations filmed inside the actual Cabinet War Rooms.

Wanted items
Crypto Museum are still looking for additional information and circuit diagrams of the scrambler systems used with the Secraphone. We are also looking for Telephone No. 710/740 and/or any of the Privacy Sets mentioned above. Crypto Museum would very much like to find a genuine Privacy Set (any model). If you can provide any of these, please contact us.
Expressions and abbreviations used on this page:
BT   British Telecom
Arguably the largest telecom operator of the UK. Previously state-owned and known as the General Post Office (GPO).

CB   Central Battery system
System in which all the energy needed for transmission and signalling is delivered by the exchange. No local batteries or hand generators are used at the telephone end. (More [3])

CBS   Central Battery Signalling system
Similar to a CB system, except that the mircophone is powered locally by a battery at the telephone end. Power for signalling is provided by the exchange as in a CB system. In the UK there were three types of CBS. (More [3])

GPO   General Post Office
The state-owned post and telecommunications operator in the UK, before it was renamed BT and privitised. The GPO was als known as British Post Office (BPO) and simply as Post Office (PO). It is currently known as British Telecom (BT).

PL   Plessey
Manufacturing code used on the body of the telephone sets and also inside, often stamped on the chassis. The manufacturer's code is also cast inside the bakelite body.

TE   TMC (see below)
Manufacturing code used on the body of the telephone sets and also inside, often stamped on the chassis. The manufacturer's code is also cast inside the bakelite body.

TMC   Telephone Manufacturing Company
The original manufacturer of the Secraphone in St. mary Cray (Kent, UK) [6].

  1. Marc Cook, WW II Secraphone
    Website. Retrieved January 2014.

  2. Henderson, WW2 Army Secraphone
    Website. Retrieved january 2014.

  3. Robert Freshwater, TELEPHONE No. 394
    BOBs Telephone File (website) Retrieved June 2014.

  4. Post Office (GPO). Index to E.I.s 1 on external construction and maintenance
    11 September 1959. Retrieved January 2015.

  5. Post Office (GPO). Index to E.I.s 1 on external construction and maintenance...
    4 April 1962. Retrieved January 2015.

  6. Telephone Manufacturing Co Ltd. (TMC), Advert for Secraphone
    Date unknown, but believed to be post-war. Retrieved January 2014.
    Via Sam Hallas, website.

  7. PO 2 Telecomms Headquarters, Automatic and Subscribers Privacy Equipment...
    ...Telephone No., 740 and Privacy Set No. 8
    Document number N5164, 12 March 1975.

  8. PO 2 Telecomms Headquarters, Automatic and Subscribers Privacy Equipment...
    ...Telephone No., 740 and Privacy Set No. 8
    Document number N5166, 23 September 1974.

  9. POTDD (GPO), Telephone No. 394
    Document number N494, issue A, 25 January 1968. First released 30 September 1937.

  10. Gildings Auctioneers, Images of Privacy Set No. 8 and telephone sets
    18 June 2013. Retrieved January 2014.

  11. Connected Earth, Wartime communications: Black and green secrecy phone...
    Retrieved January 2015.

  12. Louis Meulstee, Wireless for the Warrior
    Website. Retrieved January 2015.

  13. Imperial War Museum (IWM), Photograph of Churchill in Cabinet War Rooms
    Retrieved January 2014.

  14. Ebay, GPO Black Bakelite 300 Type Phone with Scrambler button and Green Handset
    eBay seller phased_001, item 301880390186. Retrieved February 2016.

  15. P.O.E.D. 3 , N 620, Labels 252 & 253, for use with telephones with keys
    Fist issued 20 April 1952. Last updated 19 April 1967. Obtained via [3].

  16. P.O.E.D. 3 , N 264, Telephone No. 164 (handset) diagram of connexions
    First issued 25 January 1935. Last updated 5 January 1968. Obtained via [3].

  17. Wikipedia, Churchill War Rooms
    Retrieved February 2016.

  1. E.I. = Engineering Instructions.
  2. PO or P.O. = Post Office.
  3. P.O.E.D. = Post Office Engineering Department.

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Crypto Museum. Created: Tuesday 28 January 2014. Last changed: Tuesday, 25 October 2016 - 13:41 CET.
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